The invention is related to the field of power amplifiers, particularly towards noise reduction in the power amplifiers.
In a full duplex transceiver, both the transmitter and the receiver can operate simultaneously. This results in two forms of interference. The first interference is the Tx (transmit) band power getting into the receiver. The second interference is noise generated in the Rx (Receive) band by the Tx portion of the radio, and leaking into the Rx portion of the transceiver. Both types of interference are mitigated by using a duplexer. In this instance, a duplexer is a three port, passive element consisting of a pair of bandpass filters and supporting components. One filter passes the Tx frequencies and rejects the Rx, the other passes the Rx and rejects the Tx.
As the transmit (Tx) power is not completely blocked by the duplexer, the power can enter the receiver thus causing distortion. In a filter-based duplexer, this is a measure of how well the receive (Rx) filter rejects the transmit Tx band power. The Tx filter has no effect because the leakage is in the pass-band of the Tx filter.
The second interference is caused by noise in the Rx band, generated by the Tx circuitry getting by the duplexer into the receiver. This causes a loss of sensitivity. In a filter-based duplexer, this is a measure of how well the Tx filter can reject the Rx band power. The Rx filter has no effect upon this leakage because it is in the pass-band of the Rx filter. This type of crosstalk drives the design of the Tx chain. The sensitivity required of the receiver dictates how much leakage is acceptable. The rejection of the duplexer dictates how much noise power the Tx chain can be allowed to generate.
The noise figure of an amplifier is a sum of all of its noise sources, referenced to the input of the amplifier. The noise power at the output of that amplifier is the sum of the gain of the amplifier and the noise figure. Since the sensitivity required by the Rx branch of the transceiver is set, the allowable noise power out of the Tx branch effectively sets the maximum gain of the power amplifier.
In full duplex handsets, there is another constraint. The gain setting routine discussed above relies upon the Rx noise at the input to the power amplifier be at or near the noise floor available at that temperature. The noise generated in the Tx chain previous to the power amplifier must be sufficiently rejected to not increase the noise power at the output of the power amplifier. This is accomplished by inserting a Rx band reject filter just in front of the power amplifier, in the Tx band signal path. The difficulty of building this type of Rx rejection filter is such that many handsets rely upon two filters in parallel, each filter dealing with half the signal bandwidth, adding half the pass-band to the reject band frequency. This makes each filter of the pair much easier than a full band implementation. The input signal is routed through a switch, then into one of the filter pair. After the filter, the signal is then recombined with a switch.
One prior art implementation, shown in FIG. 1, includes a Rx rejection filter in front of a two-stage power amplifier. Given that this node is at or near the noise floor, and given the amount of noise generated in the power amplifier, the amount of Rx rejection required in the duplexer can be specified. The duplexer filter that implements the required rejection has an associated insertion loss. This loss is in the Tx band, and is dependent upon the Rx band rejection. Hence, the more rejection required, the higher the loss in the Tx band.
Since the power output of the handset is constant, higher loss in the duplexer requires more power out of the power amplifier. At the high power levels required, higher power requires more current, thus, reducing battery life.
Receive band filtering between the last two stages of an Nxe2x88x92stage power amplifier can reduce the Rx band noise. There are Nxe2x88x921 interstage matching networks interposing N stage amplifiers, where N greater than or equal to 2. The interstage matching networks and stage amplifiers are electrically connected in series. The Nxe2x88x921th or ultimate interstage matching network includes a Tx band filter positioned proximate to the output of the penultimate or Nxe2x88x921th stage power amplifier.